1. Field Of The Invention
The invention relates to a method of upsetting the working of a metal slug, to a sleeve for implementing the method and to a sleeve and lid assembly for implementing the method.
2. Description Of The Related Art
Metal forgings are generally obtained by forging slugs, or billets, which are part-finished rough blanks of metal parts, generally in the form of bars, used as basic elements for forging the part that is to be obtained, their volume corresponding to the volume of the latter increased by the volume lost during forging. For example, in a jet engine, the fan discs or the compressor drums are obtained by forging metal slugs.
The invention applies particularly to the working of metal slugs resulting from powder metallurgy, but relates more generally to the working of metal slugs. It is above all recognized for the working of materials that are difficult to forge, particularly as a result of small acceptable temperature ranges.
Metal slugs originating from powder metallurgy are generally obtained by extruding a container containing the powdered material. During the extrusion, the container is forced, by a press, to pass through an orifice of a cross section smaller than its own, during which operation the material forms a dense bar. Machining the container enveloping the material—and to which it has become welded during the extrusion operation—yields the slug ready for working. Current constraints dictate a maximum diameter smaller than 300 mm, typically of the order of 230 mm, for a metal slug obtained from powder metallurgy.
Furthermore, in the field of aeronautical engineering, the safety criteria are very strict and dictate checks at all stages of manufacture. The slugs have, in particular, to be inspected, for example using ultrasound, in order to detect whether any inclusions or defects are present in the metal, as a result of cracks that have appeared during forging and possibly breaks in the finished part. The requirements governing the maximum permissible defect size in the billets as dictated by the engine manufacturers are becoming increasingly strict. Slug suppliers therefore limit the diameter of the slugs in order to be able to perform quality control using ultrasound and meet the criteria dictated by the constructors. Typically, once again, this diameter is smaller than 300 mm, for nickel-based or cobalt-based metal slugs originating from powder metallurgy.
If the finished parts of the jet engine are of large volume, then the slugs have to have a high slenderness ratio, typically in excess of 2.8, often of the order of 7 to 10, in order to compensate for their small cross section.
The term “working” is intended to cover hot deformation of a metal part in order to obtain an increase in its diameter and reduction in its length, for equal volumes. The working may be done by upsetting, that is to say by applying stress in lengthwise direction of the metal slug.
In the case of metal slugs originating from powder metallurgy, a slenderness ratio in excess of 2.8 means continuous upsetting of the slugs in order to work them so as to obtain slugs in which the ratio of length to diameter is small. The ratio is brought down to a value at which they can be forged, stamped or alternatively upset again without being contained laterally, without the risk of buckling or of imperfections being created within the fibre of the metal.
Contained upsetting means upsetting in which the slug is laterally protected, none of its surfaces being in contact with the open air. The alloys resulting from powder metallurgy require the most isothermal upsetting possible, it being typically necessary for the temperature not to drop by more than 50 or 100° C. during upsetting, otherwise deep cracks or tears will appear in the material. The operating temperature lies between the plastic deformation temperature and the melting point of the alloy, thus allowing the alloy to be forged, and is limited by a maximum value defined to ensure control over the microstructure of the alloy. Furthermore, the diameter of the worked mass must not be too small, otherwise imperfections may be created in the material. It needs to be arranged such that the slenderness ratio is below 2.8.
To achieve this, the prior art teaches cladding the slug in a steel tube, which increases its diameter and affords thermal protection. The slug and tube assembly is then upset in the open air, because it has sufficient diameter. During such upsetting, the slug and the steel tube will establish a metallic bond between them, comparable to a seized connection. It is therefore necessary, after upsetting, to machine the assembly, for example machining it on a lathe, so as to remove the steel in order to find a slug that contains only the alloy originating from the powder metallurgy. Firstly, such machining is expensive, and secondly leads to a loss in slug material. This loss of material is all the greater since, in general, the interface between the slug and the tube is relatively irregular, which means that more machining has to be done as a safety measure.
It would be desirable not to use a steel sheath. However, in such a case, it would be necessary to use very hot tooling, which would cause cracks and fissures in the slug, which would then have to be eliminated, in so far as they were accessible, by grinding.